Physics

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Inverse Square Law Calculator.

Scale a source quantity between two distances using an inverse-square relationship.

On-device calculationNo signup
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Set your values

Results update as you type.

Target value: 16

Target value

0.000000
Inverse-square ratio: 0.16

Inverse-square ratio

0.000000

Results update automatically as you type.

Use Cases

Estimate light intensity at different distances

Photographers and lighting designers can estimate how light intensity falls off with distance from a source, helping to plan exposure and lighting setups.

Example: If a light source gives 100 lux at 1 meter, what is it at 2 meters? Enter 100, 1, and 2 to get 25 lux.

Calculate radiation or sound level changes

Physicists and safety professionals can estimate how radiation or sound intensity changes with distance, aiding in safety assessments and equipment placement.

Example: If radiation is 50 µSv/h at 0.5 m, what is it at 1.5 m? Enter 50, 0.5, 1.5 to get 5.56 µSv/h.

Frequently Asked Questions

What is the inverse square law?
The inverse square law states that a physical quantity (like light, sound, or radiation) is inversely proportional to the square of the distance from the source. If you double the distance, the quantity becomes one-fourth as strong.
How do I use this calculator?
Enter the reference value (the quantity at the reference distance), the reference distance in meters, and the target distance in meters. The calculator will compute the target value using the formula: target value = reference value * (reference distance / target distance)^2.
What units should I use for distances?
Distances must be entered in meters (m). Ensure both distances are in the same unit (meters) for accurate results. The calculator does not convert units.

Tips & Common Mistakes

Tips

  • Ensure both distances are in meters. If you have distances in other units, convert them to meters first.
  • The reference value can be any unit (e.g., lux, watts, counts per second) – the calculator scales it proportionally.
  • If the target distance is less than the reference distance, the target value will be larger, as the quantity increases when closer.
  • Double-check your inputs: a small error in distance can significantly affect the result due to the square relationship.

Common Mistakes to Avoid

  • Forgetting to square the distance ratio – the formula uses (reference distance / target distance)^2, not just the ratio.
  • Using different units for distances (e.g., meters and centimeters) without converting, leading to incorrect results.
  • Entering the reference value as a percentage or fraction when it should be the actual quantity value.

Last updated: August 13, 2026